1. Field of Invention
The invention relates to a sensor system for measuring the thickness of flat material, which is moved relative to the sensor system a blown film line for manufacturing film which blown film line comprises such a sensor system, and a method for operating a blown film line comprising such a sensor system.
2. Description of the Prior Art
Sensor systems for measuring the thickness of flat material, which is moved relative to the sensor system, have been disclosed in the prior art. They are preferably used for measuring the thickness of freshly extruded film. The measured values serve for regulating the film thickness. Therefore, in this context, the term “flat material” is meant to connote predominantly film-shaped or web-shaped material, which is mostly guided past the sensor system comprising a sensor device such as a sensor head.
The measuring methods used for measuring the film thickness include optical, radiometric, inductive, and capacitive measuring techniques. However, especially when producing sensitive or sticky films, sensor systems that come into contact with the film cause damages on the latter. EP 591 239 B1 therefore suggests distancing a sensor device from the film by means of an air cushion.
The sensor device, which is disclosed in EP 591 239 B1 and is distanced from the film in such a manner, is a capacitive sensor, the two electrodes of which are mounted on a sensor head on one side of the film. Both electrodes comprise active surfaces that are facing the film. An electric field, which penetrates the air cushion between the sensor device and the film, the film itself, and the space that is located behind the film and is likewise filled with air here, is also formed between these active surfaces of the two electrodes. Here, there is known to be a dependence of the capacitance of the capacitor on the material-specific dielectric constant ∈τ of the materials (air and film material) penetrated by the electric field. A change in the thickness of the film material thus changes the capacitance of the capacitor. However, it has been seen that variations in the distance between the film and the active surfaces of the electrodes also change the capacitance of the capacitor. This influence of the distance of the film—or any flat material—also exists in the other measuring principles mentioned above such as the inductive measuring principle.
However, in blown film lines, in particular, this distance changes constantly since the film flutters, which can also result in the sensor device coming into contact with the film.
DE 195 11 939 A1 therefore suggests constantly measuring the distance between the sensor device and the film, regulating the position of the sensor device based on these measured values, and constantly adapting to the fluttering movement of the film by moving the sensor device in the radial direction of the film bubble. However, it is additionally necessary here to draw in the film using low-pressure nozzles in the boundary areas of the sensor device and thus to restrict the fluttering of the film relative to the sensor device.
Another approach for the same problem is the object of EP 801 290 B1. It is likewise suggested here to regulate the distance between the sensor and the film permanently, the measurement signal for regulating said distance being acquired by measuring the stagnation pressure between the film and the sensor device. Since the movement of the sensor device is more inertial than that of the fluttering film, it is further suggested to reduce the errors resulting in thickness measurement from variations in the distance, as follows: The actual distance between the sensor and the film at the time of thickness measurement is measured. The (erroneous) measured value of thickness is corrected based on the measured value of distance with the help of an error function.
Another possibility consists in measuring the film thickness only when the correct distance between the sensor and the film has just passed through. However, this results in irregular time intervals between the measurements.
It is clear from this explanation that the measuring devices described above are complicated, expensive, and yet error-prone.
It is therefore the object of the present invention to redress these disadvantages.